Many telephone subscribers desire to have two or three telephone line service for their business or home. In addition they wish to be able to (1) answer any one of the telephone lines at any telephone set; (2) maintain the connection with a distant party on one line while answering or placing a call at another line (commonly referred to as "placing a line on hold"); and (3) engage in a multiple party conversation in which distant parties on multiple lines can converse with one another (commonly referred to as "conferencing"). While there are key telephone systems that include line control circuits that operate in conjunction with and responsive to the actuation of the keys at associated telephone sets to provide these features, it is desirable when only two or three line service is involved to be able to provide these features without such line circuits.
L. B. Jozefowski U.S. Pat. No. 3,164,688 issued on Jan. 5, 1965, discloses a key that provides only the first two of these features. More specifically, Jozefowski discloses a multiple pushbutton key in which adjacent pushbuttons are paired and associated with an individual telephone line. A first pushbutton of each pair, referred to as a talk, pick-up, or line button, is adapted when depressed to connect the speech network of the telephone set to the associated telephone line. The second pushbutton of each pair, referred to as a hold button, is adapted when depressed to disconnect the speech network from and connect a holding bridge to the associated telephone line.
The key of Jozefowski includes a two pushbutton slide bar associated with each pushbutton pair for interlocking the pushbuttons so that either button may be depressed individually but both buttons may not be depressed simultaneously. The two pushbutton slide bar is designed so that the depression of either of the buttons results in the slide bar being rocked so as to exert an upward force on and raise the other of the pushbuttons.
Jozefowski's key also includes a three pushbutton slide bar associated with each of the adjacent pairs of pushbuttons. The three pushbutton slide bar interlocks the line buttons of the adjacent pushbutton pairs so that either of the line buttons may be depressed individually but both may not be depressed simultaneously. Similar to the two pushbutton slide bar, the three pushbutton slide bar is designed so that when one line button is depressed, the slide bar is displaced so as to exert upward force on and raise the line button of the other pair. The three pushbutton slide bar is also designed so that two such slide bars can be ganged together to interlock the line buttons of three adjacent pairs of pushbuttons. When this combination is used in a six button key, the depression of any line button results in both slide bars being displaced so as to raise any other line button. Finally, the three pushbutton slide bar is designed so that there is no interaction between it and the associated hold buttons.
As a result of the foregoing, in Jozefowski's key only one line button can be depressed at any one time and therefore the speech network of the associated telephone set can only be connected to one telephone line at any one time. Further, while all of the hold buttons can be depressed simultaneously to place one or more lines on hold, the depression of a hold button only results in the raising of its associated line button. It does not interact with or serve to raise a line button of another pair.